Kalirin is a large, multidomain Rho GDP/GTP exchange factor (GEF) of the Dbl family found almost exclusively in the nervous system. Based on genetic studies in invertebrates and analysis of genes involved in X-linked mental retardation, Kalirin is expected to play a role in axonal pathfinding and synapse formation. Our studies in rat hippocampal, cortical and sympathetic neurons showed that Kalirin plays a role in axon initiation and outgrowth, dendritic growth, and spine formation and maintenance. Kalirin has two GEF domains with specificity for different Rho family GTPases and a potential kinase domain. In addition to these catalytic domains, it has interactor domains for a variety of proteins. Kalirin expression in the nucleus accumbens and striatum is stimulated by chronic administration of cocaine, with no change in cortical Kalirin levels; Kalirin is also increased following electroconvulsive shock stimulation. The exploratory studies proposed are based on our observations that antisense-mediated elimination of Kalirin expression in organotypic slices and dissociated hippocampal neurons leads first to a reduction in linear spine density followed by simplification of the dendritic tree. Aim 1 is to generate mutant mice that will facilitate studies of neuronal development and plasticity in multiple systems. In order to avoid embryonic or early postnatal lethality, and to allow generation of tissue-specific and developmentally regulated elimination of expression, two mouse lines will be created as floxed alleles. Both will be provided to all qualified investigators without restrictions. One allele will produce the Kalirin-7 null. Kalirin-7 is the most prevalent isoform in the adult brain, with a single Rho GEF domain, terminating with a unique PDZ binding motif. Kalirin-7 appears late in development and is localized to dendritic spines. The second allele will produce the total Kalidn knockout. Both will be bred with available tissue-specific and drug-inducible Cre recombinase mice. Aim 2 will examine the biochemical, histological, and developmental consequences of tissue-specific Kalirin conditional knockouts.